The overall objective of this research project is the elucidation of the mechanisms by which human tumors xenografted to the hamster cheek pouch can attain the malignant stage of invasiveness and metastasizability, properties which are rarely exhibited by neoplasms in an alien species. In particular, we are concerned with genome interactions between host and tumor which could conceivably influence the character of the tumor. The major mechanism of this kind being investigated is that of in vivo interspecific somatic cell fusion involving human tumor and normal hamster cells, and resulting in a highly malignant cell population which, after subsequent passage, is of a predominantly hamster-like character. Other methods possibly resulting in these malignant cell populations are also being investigated.